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Ross B
does any object with mass that moves create a gravity wave? So if I wave my hand it is in fact creating gravity waves, just very small ones
Ross B said:does any object with mass that moves create a gravity wave? So if I wave my hand it is in fact creating gravity waves, just very small ones
I think "very small" VASTLY overstates the strength of that particular gravity waveRoss B said:does any object with mass that moves create a gravity wave? So if I wave my hand it is in fact creating gravity waves, just very small ones
phinds said:I think "very small" VASTLY overstates the strength of that particular gravity wave
Yeah, but that would mean he/she would have to shake his/her belly rather that a hand wave.ZapperZ said:Maybe he/she is severely overweight.
Zz.
Ross B said:does any object with mass that moves create a gravity wave?
ISamson said:Yes.
Vanadium 50 said:Huh? Where did you get that? An object that is moving at constant velocity does not create a gravitational wave.
Vanadium 50 said:Huh? Where did you get that? An object that is moving at constant velocity does not create a gravitational wave.
Consider that when an object is moving at a constant velocity (relative to what?) then there is an inertial frame in which it is not moving at all... and all frames must agree about the presence or absence of gravitational waves, so if you can demonstrate that they aren't present using one frame, then you know they aren't present using any frame.Ross B said:why does an object moving at constant velocity not not create a gravitational wave? like the wake from a boat.
You may have been misled by pop-sci presentations that speak of spacetime as a "fabric", a thing that stretches and deforms and could have properties such as elasticity. It's not.if space is elastic, as a body moves thru space it deforms space and in its wake space must snap back to its original position, that deformation of space, assuming it is not a linear deformation, would cause space itself to accelerate/decelerate/strech/compact, all other factors remaining the same?
Nugatory said:The boat is moving through the water, displacing it and creating waves in it. But that analogy doesn't work for gravitational waves because space is not a substance that you displace as you move through it.
You have to remember that GR models spacetime, not space. Space is what you get when you slice 4d spacetime up into 3d sheets. So "space when you are near a massive body" and "space after the massive body has passed" are two completely separate parts of spacetime - nothing is actually deforming. Any sensible definition of "slicing up spacetime" will give you a sequence of spaces that change geometry smoothly, but the change is the change of slice you are calling "space, now", not from any deformation.Ross B said:space "deforms" around a massive objects, and therefore any object with mass. Implicit in that statement is space "un-deforms" after the object has passed by.
Please everyone, it should not take until the 12th post to correct this.Orodruin said:Pet peeve: It is important to separate the concept of gravity waves from that of gravitational waves. They are not the same thing.
To be fair, Ibix mentioned it in post #11 while I was typing and linking.m4r35n357 said:Please everyone, it should not take until the 12th post to correct this.
A couple of posters mentioned it in passing I know, but until your post nobody corrected it.Orodruin said:To be fair, Ibix mentioned it in post #11 while I was typing and linking.
There is no such thing as "the space component". The separation into space-like slices is quite arbitrary and can be done in many different ways.Ross B said:so around a massive object as the space component of spacetime does not deform, I assume only time deforms?
No. Nothing deforms. Spacetime is the shape it is. It doesn't change.Ross B said:so around a massive object, as the space component of spacetime does not deform, I assume only time deforms?
It's been fixed in the thread title.Orodruin said:Pet peeve: It is important to separate the concept of gravity waves from that of gravitational waves. They are not the same thing.
Of course it has an effect. Those successive slices of spacetime are (or can be, anyway) different "shapes" for a reason. It's just that describing that effect as "distorting space" is hopelessly inadequate for getting any actual physics done.Ross B said:so if a massive object has absolutely no impact, of any kind what so ever, on any parameter of the space time around it - how does any other object know, at a distance, it is there?
Serway is a respectable enough textbook, but its few pages on general relativity are completely superficial: no differential geometry, no tensors, no coordinate transforms, no Einstein Field Equations. This may be because of their stated goal of introducing no math beyond first year calculus.Ross B said:Im using Serway edition 4...am I wasting my time
Orodruin said:Popular science is great for learning about science and creating interest in it, but it is rather useless for learning actual science.
I think "distorting space" makes sense if interpreted as periodically stretching and squashing of space, as illustrated here by means of the ring of freely falling particles.Ibix said:Of course it has an effect. Those successive slices of spacetime are (or can be, anyway) different "shapes" for a reason. It's just that describing that effect as "distorting space" is hopelessly inadequate for getting any actual physics done.
That animation works by making a choice about what "space" means, slicing spacetime into a sequence of spatial slices, and then presenting that sequence of slices in an animation. That's fine. But it's presenting a sequence of distinct, slightly different things. In trying to understand GR it's a mistake to see it as one thing changing, because your initial choice of slicing spacetime into space was an arbitrary one.timmdeeg said:I think "distorting space" makes sense if interpreted as periodically stretching and squashing of space, as illustrated here by means of the ring of freely falling particles.
Saying "one thing" do you mean the ring? To my understanding the source of the GW causes geodesic deviation which the "distortion" of the ring shows. Can you please elaborate a little more "because your initial choice of slicing spacetime into space was an arbitrary one." It seems I'm missing something important here.Ibix said:But it's presenting a sequence of distinct, slightly different things. In trying to understand GR it's a mistake to see it as one thing changing, because your initial choice of slicing spacetime into space was an arbitrary one.
It's the block universe again. Take each frame (in the filmography, not physics sense) of the animation and print it. Stack up the printouts. Now dissolve the paper and leave the ink. This is the block universe and the wiggly columns of ink are the worldtubes of the dots.timmdeeg said:Can you please elaborate a little more "because your initial choice of slicing spacetime into space was an arbitrary one."
What about curvature? It's not zero in the neighborhood of a mass. I think I "get" that space-time doesn't "evolve" because time is included from the start and I also get vacuum is "nothing" part. I spent a lifetime viewing electromagnetic fields as dynamical why is this now verboten for curvature?Ibix said:So there's no sense of "elastic deformation of a fabric".
Paul Colby said:I spent a lifetime viewing electromagnetic fields as dynamical why is this now verboten for curvature?
Yes. I just saw that also Wikipedia talks about distortion: " As a gravitational wave passes through the particles along a line perpendicular to the plane of the particles (i.e. following the observer's line of vision into the screen), the particles will follow the distortion in spacetime ". This misleads to believe that space is something physical which can be stretched ect. In deed, as in the case of the expanding universe one should think of changing distances instead.Ibix said:To make the animation you need to put the paper back in. The paper is your choice of what "space" means. And in GR, as in SR, there's no reason to prefer horizontal or sloped pieces of paper. It's an arbitrary choice. Furthermore, the pieces of paper aren't expanding or contracting. Each one has a larger or smaller scale compared to its neighbour, but nothing is changing - unless you mistake the sequence of similar things for a single thing changing.
PeterDonis said:But if you're willing to accept that drawback, the procedure itself is perfectly legitimate.
Paul Colby said:this from a space-time view nothing is dynamical seems like a vacuous viewpoint. Come rain or shine one still must solve a system of hyperbolic equations to understand the physics of an isolated or cosmological system
PeterDonis said:it's important to know that they mean something different by the word "dynamical" than you do
Paul Colby said:it's certainly fair to ask what is this new meaning of the word "dynamical" of which space-time is not